A high illumination, low contrast scene is for example occurring when an airplane is landing in the mist. In such a scene the illumination level of a pixel of an image sensor is high and therefore the charge storage requirements of such a pixel are high. However, the same sensor may be used in a low-light situation, where the charge storage requirements are low. Thus pixels with a high dynamic range are required.
FIG. 1 shows a schematic cross-section of a prior art 4T pixel. The illustrated pixel 100 comprises a pinned photodiode 110, a transfer gate 120, a floating diffusion 130 and a reset gate 140. The dynamic range of the 4T pixel is thereby dependent on the storage capacity of the floating diffusion.
In order to increase the storage capacitance of optical sensors, Sugawa proposes in US 2009/0045319 to configure these optical sensors such that photocharges, overflowing from the photodiode, are stored in a plurality of storage capacitance elements. Thereby an optical device can be obtained which maintains a high sensitivity and a high S/N ratio and has a wide dynamic range. Yet these capacitors consume area and thus reduce the fill factor and/or increase the pixel size.